Igniter devices are used commonly in applications such as airbag deployment and seat belt pretensioner activation in cars. The igniter device comprises an igniter element which converts electrical energy to heat. Typically, the igniter element comprises a hot wire bridge which is heated by a current of 1 Amp (A) or more. In, for example, airbag applications, the heat generated in the igniter element ignites a pyrotechnic material adjacent the heater element which burns a propellant. This produces gas to inflate the airbag.
A particular concern to automotive manufacturers is the possibility of igniter devices firing inadvertently due to a fault. For example, inadvertent firing of an airbag igniter device could have serious consequences for a driver whilst driving. Two of the major type of faults which can occur in igniter devices are: 1) improper connections to either side of the igniter element; and 2) faulty igniter elements themselves. It is therefore common to have both high and low side switches coupled to the igniter element by wires to ensure that a fault, such as improper connection to the igniter element, does not fire the igniter device.
However, the igniter device may still be inadvertently fired due to Electrical Static Discharge (ESD) and/or RF energy induced in the long wires between the high and low side switches and the igniter element. This problem can be mitigated by using a hot wire bridge igniter element which has a low resistance of 2 ohms and 1.5 Amps (A) for 3 ms firing conditions. That is, 13 milli-Joules (mJ) of energy is required to fire the hot wire bridge. The required firing energy is thus relatively high compared to the typical ESD and RF induced energies and thus this solution provides some protection against induced currents. However, in view of the increase in RF sources such as GSM radios, repeaters and electromagnetic interferences, automotive manufacturers consider that this form of protection is not sufficient.
A further disadvantage of using a low resistance igniter element is that the high and low side switches have to supply a current of 1 A or more and therefore require the use of oversized MOSFET power transistors which have an on-resistance R.sub.dson in the range of 2 ohms, like the igniter element itself. Such high and low side switches are therefore expensive and require large die area. Furthermore, about 66% of the available energy is lost through the high and low side switches which means that the efficiency of the firing loop is relatively low. Under these conditions, the level of energy that must be stored in the reservoir capacitors, which are used to fire an airbag igniter element should the battery be disconnected or shorted during a crash, is very high.
Airbag igniter devices are safety devices which are not intended to explode in normal driving conditions but must explode when fired due to an accident. It is therefore necessary to make regular diagnostics to ensure that the igniter is able to work in case of an accident over the given lifetime of the airbag equipment (typically 15 years). In fact, airbag manufacturers are now requiring that diagnostics be provided for all airbag igniter devices.
A typical diagnostic system uses current limited voltage sources or current sources to test for improper connections to either side of the igniter element. A diagnostic current which is less than the current required to fire the igniter element is applied and the voltage at one or the other end of the igniter element is measured to check for shorts to battery, shorts to ground and open firing loops. Typical diagnostic currents are in the range of 15 to 30 mA. By measuring the diagnostic voltage across the igniter element for a given diagnostic current, it is also possible to determine the resistance of the igniter element and whether the resistance changes. If the resistance of the igniter element changes by too much, the igniter device may not fire and a warning signal can be generated by a processor.
As mentioned above, in order that the hot wire bridge igniter element is not fired in error due to ESD and RF induced currents, the resistance of the hot wire igniter element is chosen to be relatively small at about 2 ohms. However, this means that with diagnostic currents in the range of 15 to 30 mA, the diagnostic voltages to be measured are relatively small (less than 60 mv). Thus, it is difficult to determine the voltage and hence the resistance of the igniter element accurately. Precision measurement circuits may be used but this increases cost, complexity and size of the diagnostic circuit.
Some of these problems can be overcome using a semiconductor igniter element in place of a hot wire bridge igniter element. U.S. Pat. No. 4,843,964 and French patent application no. FR 9409894, each describes a semiconductor igniter element comprising an electrical material (such as a highly doped semiconductor) formed as a bridge of small size and extending between two spaced conductive pads. Such semiconductor igniter elements are also known as semiconductor bridge igniter elements. Semiconductor igniter elements have advantages over hot wire bridge igniter elements since they have a more narrowly defined firing current or energy and they become open circuited once fired.
The use of a semiconductor igniter element in an igniter device will improve and reduce the cost of the igniter device since the firing current of the semiconductor igniter is narrowly defined and the level of energy required to fire the device is reduced (less than 1 mJ). These improvements also mean that the size of the high and low side power transistors can be reduced and also the level of the energy stored in the reservoir capacitor. However, the sensitivity of such a device incorporating a semiconductor igniter to external energies, such as ESD and RF, increases.
There is therefore a need for a `smart` igniter which igniter is only fired by a specific code and has increased insensitivity to external energies.
Development of smart igniters for mining applications has been going on for some time. The purpose of using encoded smart igniters in mining applications is to build a network of igniters and sequentially fire the igniters using a computer, see for example U.S. Pat. No. 4,819,560. The kind of devices which have been developed for mining applications are relatively complex, use at least 3 wires per device, need too much energy and do not fulfil the automotive specifications in terms of package, size, compatibility with existing solutions and cost. Another drawback is that mining igniter devices are not designed as safety devices and are produced to be fired at 100% in a relatively short time after production which are opposite requirements to automotive igniter devices
U.S. Pat. No. 5,225,986 discloses an igniter system having a hot wire bridge igniter element, an electronic lock and a controlled switch. The igniter element is only fired after transmission of an unlocking code which corresponds to a specific code of the electronic lock. Such a system is therefore substantially immune to the affects of external energies such as ESD and RF. However, the igniter system disclosed in this U.S. patent has no diagnostic circuits and suffers from the disadvantages outlined above with respect to a low resistance hot wire bridge element.
There is therefore a need to provide an improved control circuit for controlling the firing of an igniter element wherein the above problems and disadvantages are mitigated.